You could put one in series with a fixed value cap (NP0 perhaps for
stability) to bring down the lower value. E.g., a 100 pF cap would give a range
of about 49-80pF, if I have punched the buttons on my calculator correctly. I
have often done this to modify the range of a variable cap I have on hand. I am
sure in a very critical application this could present problems, say if the
currents were relatively high and the cap shifted due to heat.